A fluorimetric and colorimetric dual-mode sensor based on N, S co-doped carbon dots functionalized silver...

A fluorescent-colorimetric dual-signal platform, N, S co-doped carbon dots functionalized silver nanoparticles (NS-CDs-AgNPs), was designed in situ by reducing AgNO₃ in the presence of N, S co-doped carbon dots (NS-CDs) under the assistance of microwave irradiation for glucose determination. With the formation of silver nanoparticles (AgNPs), the intrinsic fluorescence of NS-CDs was quenched, showing the fluorescence state was off. Whereas the fluorescence of NS-CDs can be switched on when a trace amount of H₂O₂ was added. Based on this novel phenomenon, the peroxidase-like activity of NS-CDs-AgNPs by using 3,3',5,5'-tetramethylbenzidine (TMB) chromogen and H₂O₂ as substrates was evaluated. The K_m values of the prepared probe for H₂O₂ and TMB were 0.84 mM and 0.01 mM with the V_m of 6.65 × 10^-8 M S^-1 and 3.01 × 10^-8 M S^-1, respectively. The results showed that NS-CDs-AgNPs had good peroxidase-like activity and strong affinity to TMB and H₂O₂. It confirmed that there is a redox interaction between AgNPs and H₂O₂, and H₂O₂ can oxidize Ag to produce Ag⁺, which is the main reason that the fluorescence of NS-CDs-AgNPs can be activated by H₂O₂. The hydroxyl radical (·OH) was formed in the process of reaction, which can further oxidize TMB for color reaction. Meanwhile, glucose can be oxidized to produce H₂O₂ in the presence of glucose oxidase (GOx). Based on the phenomenon, a fluorimetric and colorimetric dual-mode sensor for glucose detection was established. Satisfactory results were obtained with the linear range of 0.1-80 μM for fluorimetric mode and 0.5-5 μM for colorimetric mode, respectively. Additionally, the LOD was below 0.32 μM and 0.21 μM, respectively. The method was successfully applied to determine the glucose in human serum with satisfactory recovery and RSD.